Margaret, often called Margaret of Constantinople, ruled as Countess of Flanders during 1244–1278 and Countess of Hainaut during 1244–1253 and 1257–1280. She was the younger daughter of Baldwin IX, Count of Flanders and Hainaut, and Marie of Champagne.
Called the Black due to her scandalous life, the children of both her marriages disputed the inheritance of her counties in the War of the Succession of Flanders and Hainault.